First, we should note that China should not really be characterized as a superpower at this point in its history. True, it has the second biggest economy in the world at this point. However, this is largely because of the size of its population. China’s overall economy is huge, but on a per capita basis, it is very small. China is poorer, on a per capita basis, than such countries as Ecuador, Thailand, Peru, Colombia, and Kazakhstan. This reduces the extent to which we can call it a superpower. So does the fact that its military is not tremendously powerful outside its own borders. For example, China has just launched its first aircraft carrier (which it did not even build itself), meaning that it would have a very hard time projecting power anywhere overseas. Again, this is not the mark of a superpower.
That said, let us look at your question as if China truly were a superpower. The first cause of this is the simple fact of China’s population. A country with over 1 billion people has a head start in gaining power. A second factor is geography. China is a very large country that has been, at least since World War II, relatively immune from any danger from the outside world. This has allowed it to build strength. Third, China was lucky to have Deng Xiaoping emerge as the first major leader after the death of Mao Zedong. Deng pushed China to modernize its economy and the results of his efforts have so far been quite impressive. Finally, China emerged at a good time. The West was already rich and therefore could be a market for exports. Countries like Japan and South Korea had taken their turn as the source of cheap exports to the West and had now grown too rich to fill that role. China emerged as a viable replacement for those countries.
These factors have helped China become more economically and militarily powerful. It now remains to be seen if China can move up to true superpower status in economic and military terms.